Our objective is to find methods of manipulating the diet of an individual, as opposed to a population, in order to reduce the risk of neoplasia. The work proposed is based on the following observation: When non-inbred rats are given freedom of dietary choice, the quantity and composition of the diet selected varies from animal to animal in a way that greatly increases the risk to the individual of developing a neoplasm. The individual specificity in dietary condition that favor oncogenic processes suggests that among conventionally fed animals, those individuals that develop tumors under one set of conditions would have a lower risk of developing tumors if they were maintained under different dietary conditions. The studies in this application have been designed to establish whether or not knowledge of each individual's diet can be used to decrease the risk of spontaneous neoplasia by dietary manipulations, and, to what extent genetic factors determine the optimal tumor-promoting diet of each individual. The dietary preference exhibited by an animal early in postweaning life characterizes the dietary habits throughout much of the animal's lifetime. This offers a basis for selecting the dietary regimen to be imposed for regulating tumor susceptibility. Other studies are designed to determine the extent to which the "tumor inhibiting" dietary regimen voluntarily adopted by animals permitted freedom of choice will depress tumor incidence when that regimen is used as a guide for feeding other animals.